In the double-slit experiment an electron that is not being watched behaves as if it took both paths. Physicists call such a blend a superposition. For an electron, people learned to live with it. Erwin Schrödinger, who had written the central equation of quantum mechanics in 1926, asked an awkward question nine years later: if the theory is complete, why does the blending stop at small things?

In a 1935 article in the German journal Die Naturwissenschaften, Schrödinger described what he called a “quite ridiculous” case. Seal a cat in a steel chamber with a device the cat cannot touch. The device holds a speck of radioactive material so small that in one hour there is an even chance one atom decays and an even chance none does. If an atom decays, a Geiger counter fires, a relay drops a hammer, the hammer breaks a flask of poison, and the cat dies. If not, the cat lives.

Quantum theory describes the unobserved atom after an hour as a superposition of decayed and not decayed. Follow the chain of the apparatus and the same mathematics describes the whole box as a superposition of dead cat and living cat, smeared together in equal parts, until someone opens the lid.

Schrödinger did not believe this and did not want his readers to. His point was that a vagueness tolerable inside an atom becomes, by an ordinary chain of cause and effect, a vagueness about a cat, and nobody has ever seen a blurred cat. Something in the standard account had to be incomplete. He had been trading letters that summer with Einstein, who made the same complaint using an imaginary keg of gunpowder that would be both exploded and unexploded.

The cat dramatizes what is called the measurement problem. Quantum mechanics has two rules that do not sit comfortably together. Left alone, a system’s wave function evolves smoothly and holds all possibilities at once. When measured, it gives one definite result, with odds the wave function supplies. The theory does not say what counts as a measurement or where the changeover happens. Is it at the Geiger counter? The cat? The scientist? Her colleague in the hall, waiting to hear?

That last version is real. In 1961 Eugene Wigner imagined a friend who opens the box while Wigner waits outside the closed laboratory. To the friend the cat is definitely one thing. To Wigner, the friend and the cat are together in superposition until he opens the door.

Copenhagen. The view associated with Niels Bohr and Werner Heisenberg treats the wave function as a tool for predicting what instruments will show. Large instruments are described in ordinary terms, and asking what the cat “really” is before the reading is not a question the theory undertakes to answer. The Geiger counter is already a measurement. The cat is alive or dead well before the lid opens.

Many worlds. In 1957 Hugh Everett, a Princeton graduate student, proposed taking the equations at face value. Nothing ever collapses. The universe’s wave function branches, and there is a version of the observer who finds a living cat and a version who finds a dead one, each unaware of the other.

Decoherence. Beginning with H. Dieter Zeh in 1970 and Wojciech Zurek in the 1980s, physicists worked out why we never see blurred cats whichever interpretation is right. A cat is warm, and collides with air and light trillions of times each instant. Each collision carries away information about its state. The delicate link between “alive” and “dead” is scrambled into the environment in far less than a billionth of a second, leaving what looks for all purposes like one or the other.

Something new. A minority suspect quantum mechanics itself needs changing, with collapse as a real physical process that becomes rapid for massive objects. Roger Penrose has proposed gravity as the trigger. Experiments with ever-larger objects are testing such ideas.

Nobody has put an animal in superposition and nobody is trying to. But “cat state” is now a working term in laboratories for a system placed in two clearly distinct states at once.

The same physics now has a job. The bits of a quantum computer, called qubits, are small engineered superpositions, and some designs are literally named “cat qubits”. Keeping decoherence away from them is the main engineering problem in the field.

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